Ligand Field‐Induced Dual Active Sites Enhance Redox Potential of Nickel Hexacyanoferrate for Ammonium Ion Storage

Enhancing the redox potential of cathode materials is vital for increasing the energy density of aqueous ammonium ion batteries (AIBs). Prussian blue analogues (PBAs), with their inherently high redox potentials and open frameworks, are promising candidates. However, further boosting their redox potential and understanding their NH₄ ⁺ ion storage mechanisms remain critical challenges. In this work, a novel ligand field-induced dual active sites mechanism is introduced by incorporating Ni into the PBA framework, activating Ni as an additional redox center for NH₄ ⁺ ion storage. The electron transfer from Ni to Fe within the Ni─N≡C─Fe chain enhances the redox potential and electrochemical performance of nickel hexacyanoferrate (NiHCF). For the first time, the electrochemical activity of Ni is demonstrated in NiHCF during NH₄ ⁺ ions intercalation and de-intercalation. NiHCF exhibits elevated redox potentials, superior rate performance, and robust cycling stability compared to iron hexacyanoferrate (FeHCF). Advanced characterization techniques and density functional theory calculations confirm the activation of Ni and the enhanced interaction between NH₄ ⁺ ions and the framework. These findings provide new insights into the NH₄ ⁺ ion storage mechanism of PBAs and offer a promising strategy for designing high-energy-density cathode materials for AIBs.